Transparent glass and glass-ceramic microspheres (i.e., beads) are used as optical elements for retroreflective sheeting and pavement markings. Such microspheres can be produced, for example, by melting methods. Such melting methods may include melting a raw material composition in the form of particulate material. The melted particles can be quenched, in air or water for example, to give solid beads. Optionally, quenched particles can be crushed to form particles of a smaller desired size for the final beads. The crushed particles can be passed through a flame having a temperature sufficient to melt and spheroidize them. For many raw material compositions this is a temperature of about 1500° C. to about 2000° C. Alternatively, the melted raw material composition can be poured continuously into a jet of high velocity air. Molten droplets are formed as the jet impinges on the liquid stream. The velocity of the air and the viscosity of the melt are adjusted to control the size of the droplets. The molten droplets are rapidly quenched, in air or water for example, to give solid beads. Beads formed by such melting methods are normally composed of a vitreous material that is essentially completely amorphous (i.e., noncrystalline), and hence, the beads are often referred to as “vitreous,” “amorphous,” or simply “glass” beads or microspheres.
One exemplary patent that relates to microspheres is U.S. Pat. No. 6,335,083 (Kasai et al.), that relates to transparent solid, fused microspheres. In one embodiment, the microspheres contain alumina, zirconia, and silica in a total content of at least about 70% by weight, based on the total weight of the solid, fused microspheres, wherein the total content of alumina and zirconia is greater than the content of silica, and further wherein the microspheres have an index of refraction of at least about 1.6 and are useful as lens elements.
Other exemplary patents include U.S. Pat. Nos. 6,245,700 and 6,461,988 (Budd et al.) that relate to transparent, solid microspheres that contain titania plus alumina, zirconia, and/or silica in a total content of at least about 75% by weight, based on the total weight of the solid, microspheres, wherein the total content of alumina, zirconia and titania is greater than the content of silica.
Although, the microspheres of U.S. Pat. No. 6,335,083 (Kasai et al.) and U.S. Pat. Nos. 6,245,700 and 6,461,988 (Budd et al.) exhibit sufficient transparency and mechanical properties for use as retroreflective lens elements for retroreflective articles such as pavement markings, industry would find advantage in microsphere compositions having improved properties and methods of making such microspheres.